Ferric sulfate (Fe.sub.2 (SO.sub.4).sub.3) is a commonly used material of commerce. It finds use in, for example, potable water and sewage treatment processes, where it acts as a flocculant to remove suspended particles in the water. Furthermore, ferric sulfate is a useful precursor to other commonly used materials of commerce. For example, roasting of ferric sulfate yields ferric oxide and sulfur trioxide gas, while hydration of sulfur trioxide gas yields sulfuric acid.
Ferric sulfate has been produced from many materials under a wide range of conditions. For example, various metal-containing byproduct streams from smelting ("flue dust"; see, e.g., U.S. Pat. No. 2,252,332) and steel manufacture ("baghouse dust"; see, e.g., U.S. Pat. No. 5,286,465) may be reacted with sulfuric acid to generate ferric sulfate. However, there is a need in the art for a more efficient, less expensive ferric sulfate manufacturing process.
In addition, there is a need in the art for processes that convert or otherwise consume chemical byproduct streams to provide useful materials under economical conditions. For example, ore roasting was once a very widely used process for metal recovery. However, ore roasting generates various pollutants, such as sulfur dioxide, and therefore this process is in disfavor. The lixiviation or leaching (i.e., selective dissolution) of metal bearing mineral materials by hydrometallurgical techniques has become a popular alternative to ore roasting, because roasting gases such as sulfur dioxide are not normally produced in leaching processes. In a typical leaching process, the feedstock having the metal(s) of interest is exposed to reactive solutions, thereby achieving a purification of the feedstock.
However, ore leaching also suffers from certain drawbacks, for example, the leaching solutions have to be disposed of and/or utilized in some way. The leaching solutions are typically acid-containing and corrosive, thus are difficult to dispose of and are expensive to replace. In addition, the leaching solutions often contain valuable metals, which are desirably recovered. Electrowinning is a process whereby leaching solutions can be treated to isolate the valuable metals therefrom. However, when iron is present in the leaching solution, it causes problems for the electrowinning process, particularly when the iron concentration is greater than about 3 g/liter.
A traditional approach to recovering useful metals from iron-containing acidic aqueous leaching solutions is through addition of an alkali to the leaching solution, to thereby precipitate ferric ions as hydroxides. However, this conventional precipitation method requires the separation of hydroxides, which are difficult to filter from the liquid phase.
There is thus a need in the art for improved processes to produce ferric sulfate, and processes that can remove iron from leaching solutions to allow these solutions to be used in electrowinning. The process of the present invention fulfills each of these needs, and provides other related advantages.